I can thing of a couple of possibilities. The battery may in fact be bad is such a way that it can't supply the current needed to run the machine by itself. Another possibility is that the laptop power supply on the motherboard is broken such that it can charge the battery but not draw power from it. Toshiba should have a battery diagnostic you can run, but it likely requires you to unplug external power. There are certainly other possibilities but if you don't want to take it to a shop, you can try replacing the battery. If you have to open the machine to do that, use great care and anti-static (ESD) precautions. It is easy to break something. Of course, you may find you wasted your money on a replacement battery. How old is the machine? If it is several years old, they battery is likely about worn out anyway.